In April 1980, a cross-sectional survey was made of the prevalence of dental fluorosis and dental caries in seven communities in Illinois that had different concentrations of naturally-occurring fluoride in their public water supplies. The communities were grouped into four categories according to the approximate relation of their actual fluoride concentration to the recommended optimal fluoride concentration for the area (1, 2, 3 or 4 times optimal). The study population consisted of 807 school children, ages 8-16, who had lived continuously from birth in their respective communities and had always used the community water supply as their primary source. Dental caries was assessed with the DMFS index and dental fluorosis was measured with Dean's Index and with a newly developed Tooth Surface Index of Fluorosis (TSIF). Fluorosis was assessed independently in each child by each Index. In addition, color photographs of the teeth of some children were taken to depict varying degrees of fluorosis. In April 1982, 317 children from four communities in Iowa with negligible concentrations of fluoride in their drinking water were examined for comparison with the children in Illinois.